A panel of a high-density wiring board (hereinafter referred to simply as a board), single-piece substrates (substrates cut from a panel) arranged on a tray, wafer, or the like is used for a semiconductor product. This kind of board or the like is about 200 to 400 mm square in a planer direction for example. The microelectrodes called bumps and arranged on the board is however only about 0.02 to 0.1 mm in height and height dimension inspection of the bumps is regarded as important. Measuring accuracy required for the height inspection of the bumps is on a micron level, meaning that it is required to measure an object having a large surface area with high accuracy.
Besides, since semiconductors are mass-produced articles, markedly high speed is also required even more than high-accuracy measurement.
Two methods may be taken for taking high-speed, high-accuracy shape measurements of a surface of an object having such a large area have been put to practical use. The first method may be a method which takes measurements by using a surface shape measuring instrument of an area measurement type and moving the object or measuring instrument step by step (hereinafter referred to simply as a step scanning type). The second method may be a method which takes measurements by using a surface shape measuring instrument of a linear measurement type and continuously scanning the object or measuring instrument (hereinafter referred to simply as a continuous scan type).
The step scanning type, which uses the surface shape measuring instrument of the area measurement type, does not require any step movement as long as the entire object fits in an area size, i.e., a field-of-view size. However, when high-accuracy measurement is required, it is difficult for the measuring instrument to have a sufficiently wide field-of-view size due to constraints of an optical system and constraints of a resolution of a two-dimensional detector. Consequently, as the field-of-view size is generally a few millimeters square to a few tens of millimeters square, it is necessary to piece together measurements taken in a large number of fields of view via step movements.
In the case of step movements, it is necessary to move fast and stop quickly for high-speed measurement. The vibrations during acceleration and deceleration affect measuring accuracy, making it necessary to increase apparatus rigidity and resulting in size and cost increases of the apparatus.
Meanwhile, in the case of the continuous scan type, velocity stability and high linearity are required of a movable stage, but there is not much acceleration and deceleration, and thus high-speed movement can be achieved without incurring size and cost increases. Thus, it can be said that the continuous scan type is superior as a method for performing shape measurement of an object having a large area with high accuracy at high speed.
As a surface shape measuring technique of the continuous scan type, a light section method is used commonly. The light section method which is based on a principle that, when linear pattern is projected onto an object and a straight light beam projected from a direction different from a projection direction of the linear pattern is observed on an image, a straight line appears distorted according to undulation of the object, is a technique for measuring an amount of distortion on the image and thereby measuring the undulation, i.e., surface shape. One line of surface shape on the linear pattern can be measured from one image, and by acquiring images continuously while moving the movable stage continuously, it is possible to measure the surface shape of the object seamlessly.
However, even though the entire object can be measured in a scanning direction, in a direction orthogonal to the scanning direction, since scanning is limited by the field-of-view size of the image, each time a scan is finished, a scan may be carried out again by moving one field of view using the movable stage having an axis orthogonal to the scanning direction, and this may be repeated until scanning of the entire object is finished. That is, movements along two axes are necessary using a stage along a scanning axis and a stage along a field-of-view movement axis.
With a typical inspection apparatus, a measuring object is mounted on an XY stage and measurements are taken by scanning the XY stage. A configuration of a typical inspection apparatus is shown in FIG. 7. When a measuring object 704 is a mass-produced article such as a semiconductor product, a large quantity of the measuring objects 704 needs to be measured and it is necessary to take measurements by replacing the measuring objects 704 one after another. Therefore, transport apparatus (loader 705 and unloader 706) are provided to replace the measuring object 704 on an XY stage 702 and a Y-axis stage 703. Typical operation will be described below.
When the measuring instrument 701 completes the measurement of the measuring object 704 on the XY stage 702, an elongate Y-axis stage 703 moves to a location where the measuring object 704 can be replaced without interfering mechanically with the measuring instrument during the replacement. Subsequently, the product already measured is removed by the transport apparatus and a product yet to be measured is placed, positioned, and fixed on the XY stage 702 and 703. The product is moved to an original position under the measuring instrument by the Y-axis stage 703 and measurement is started again. This operation is repeated.
Inspection of mass-produced articles requires high rapidity. Therefore, a surface shape measuring instrument of the continuous scan type advantageous to high-speed measurement is used as described above. However, even if high-speed measurement is achieved, if it takes time to replace the product on the XY stage, this cannot be said to be a high-speed inspection when all things are taken into consideration.
It can be said that product replacement time, which not only affects apparatus speed, but also keeps the measuring instrument idle, creates an uneconomical situation in which the expensive measuring instrument is not fully utilized.
Also, although the light section method is used as a surface shape measuring technique of the continuous scan type, since only one line of surface shape measurement results is obtained from one image, the light section method cannot be said to be an effective technique actually. To compensate for such inefficiency, measures are taken to improve efficiency, such as increasing image sensing speed of an image sensing device and processing images using a dedicated electronic circuit installed near the image sensing device without transferring the images to a computer, though such special development is desirably be unnecessary if possible. The efficiency can be said to be poor also in terms of development efficiency.
Furthermore, detection of a center position of the linear pattern by the light section method is considered to be affected by surface conditions of the object in principle, and thus it cannot be said that reliability of measurement is high. Furthermore, a laser beam normally used as linear pattern becomes a major error factor in center position detection because the laser beam is prone to speckles which produce an unnatural intensity distribution in the image formed on the image sensing device.